Power operators for garage doors and the like generally comprise reversible electric motors connected to the garage door by means of a chain and pulley arrangement and provided with suitable controls to stop the motor at predefined limits of door travel. The typical operator comprises a case made out of stamped metal parts which are fastened together with machine screws, nuts and bolts, and the like, a small electric motor, a speed reducer, some kind of clutch, and a chain and sprocket or pulley drive system.
In one prior art system shown in U.S. Pat. No. 2,672,582 the drive motor is caused to be shut-off at the up and down limits of door travel by sensing the fact that the door encounters a mechanical obstacle; e.g., the floor of the garage or the top limit of the door carriage. Motor shut-off is accomplished by means of a camming clutch arrangement on the motor output shaft which shifts a collar axially along the output shaft to trip a shut-off switch. More specifically, this is provided by means of a pair or collars having a ramp-like meshing combination of cam surfaces, one collar being fixed on the output shaft and the other being loosely disposed on the output shaft. When the door encounters a mechanical obstacle, the motor continues to rotate the fixed collar, but the loose collar which is connected to the door begins to slip and the cams cause an axial shaft of one collar relative to the other. When the free collar slides away from the fixed collar, a switch is thrown to stop the motor.
There are a number of disadvantages to this prior art system including the fact that the clutch arrangement is operative for every half cycle of door travel and thus must be of extremely durable construction to avoid deterioration in an unacceptably short time. Even with a durable clutch, the operation of stopping the door by running it against a mechanical stop at full speed is bound to produce a good deal of wear on the door, the motor, and the various other components of the operator. Perhaps most importantly is the fact that such a device has the capability of trapping a small child under the door until help arrives to restart the motor in the opposite direction. Finally, the prior art system described above fails to provide for the adjustment of the limit stops.
A more practicable approach to the provision of limit stops is disclosed in U.S. Pat. No. 2,951,920. The system disclosed in that patent provides for a small threaded shaft which is rotated in synchronism with the drive shaft of the operator and which is further provided with a pair of traveler nuts which are held against rotation and which thus travel linearly along the threaded shaft as the output shaft of the operator rotates. The system futher comprises a pair of limit switches which are actuated by the fixed traveler nuts so as to stop the motor at each of adjustable up and down limits of travel. Thus the door is stopped by opening a motor control switch and not by running the door against a mechanical stop.
In addition, the feature of obstacle detection is best provided by means for causing an instant reversal of the direction of motor shaft rotation such that the motor does not merely stop, but actually backs away from the obstacle. This approach, however, calls for a mechanism to disarm the instant reverse just before the door reaches bottom so that the ordinary closing function can not produce a reversal.
The invention of the disclosure herein relates to systems of the second type described above; i.e., a garage door operator of the type providing adjustable limits along with an obstacle detector of the instant reverse type.